Reports

Hydrogeology of the Patuxent aquifer system in the Waldorf area, Charles County, Maryland

Four test wells were constructed in the Waldorf area of north-central Charles County to (1) provide basic geological and hydrogeological data necessary to help characterize the Patuxent aquifer system, and (2) begin to provide long-term water-level data to help determine the effects of existing regional and potential local withdrawals from the Patuxent aquifer system.

The top of the Patuxent aquifer system in the Waldorf area as determined from the test wells ranges from 1,002 to 1,633 feet below sea level. Total thickness of the Patuxent aquifer system ranges from 285 to 440 feet, and sand percentage ranges from 30 to 40 percent of total thickness. Sands in the Patuxent aquifer system in the Waldorf area are thinner, less frequent, and finer-grained than in more productive areas to the west in the Bryans Road - Indian Head area and to the north in Prince George’s and Anne Arundel Counties.

Transmissivity of the Patuxent aquifer system as determined from the test wells ranges from 77 to 335 feet squared per day for the pumping phases of the tests and from 106 to 618 feet squared per day for the recovery phases of the tests.

Water levels measured in the test wells during November of 2014 range from 20 to 42 feet below sea level. Water levels are declining at a rate of 3 to 6 feet per year. These declines are occurring in the absence of any local pumpage from the Patuxent aquifer system, and are likely a response to withdrawals in the Bryans Road - Indian Head area to the west or the Chalk Point Power Plant to the east.

Water samples collected from the test wells were a sodium-bicarbonate type, with total dissolved solids ranging from 269 to 605 milligrams per liter. None of the samples exceeded the U.S. Environmental Protection Agency’s Primary Drinking Water Standards. Secondary standards were exceeded for iron, total dissolved solids, aluminum, and color in one or more of the wells. Polonium-210 concentrations were all less than 2 picocuries per liter.

There is more than 760 feet of available drawdown in the Patuxent aquifer system in the Waldorf area; however, use of the aquifer may be constrained by deep drilling depths, declining water levels, and relatively low transmissivity.